[Tinyos-help] ADC

[Andrew]

Hi there,

You'll probably find the input impedance presented by the ADC3 input is 
not a short to ground, but a low enough to discharge of the capacitor 
fairly rapidly. In my limited experience, I have found the ADC input 
impedance to be fairly low, and it can affect the values captured by the 
ADC if the output impedance is not small enough on the device that you 
are connecting to the ADC input. One work around is to buffer the source 
you wish to sample. You can use an opamp to do this. The opamp presents 
a high input impedance (10^12 on some FET opamps) and would result in 
virtually no loading to your source. Opamps typically have a low output 
impedance meaning that when provided a suitable load (in your case an 
ADC input) the voltage presented should remain true to the input 
(provided this is a straight buffer). I gladly accept any corrections!!

Hope this helps,
Andrew



simone.s83 at libero.it wrote:
> Hi i use tmote and tinyos 1.1 ; i must use ADC to sample a voltage on Capacitor but i have a problem. In fact i use ADC3 and GND(of ADC) but this pin result short-circuit when tmote is on. This is a big problem for me because the capacitor discharge and i can't mesaure the really value. How can i resolve my problem??
>
> Thanks
>
>
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